1. Field of the Invention
The present invention relates to a phase-change optical recording medium comprising a phase-change recording material of which optical constant is changeable by the application thereto of electromagnetic wave, in particular, a semiconductor laser beam, so as to carry out the recording and reproducing of information.
2. Discussion of Background
The phase-change optical recording medium is one of the optical recording media that can recording, reproducing and erasing information by the application of a laser beam thereto.
In terms of the structure of layers, a representative example of the phase-change optical recording medium is shown in a single figure. As shown in the figure, a lower heat-resistant protective layer 2 a recording layer 3, an upper heat-resistant protective layer 4 and a light reflecting and heat dissipating layer 5 are successively overlaid in a substrate 1 in this order in the phase-change optical recording medium.
A variety of materials, for example, GeTe, GeTeSe, GeSbTe and GeSbTeSe are conventionally known as the phase-change recording materials for use in the recording layer 3. Furthermore, a phase-change recording material comprising AgInSbTe which exhibits high sensitivity and excellent erasing properties is disclosed in Japanese Laid-Open Patent Applications 2-37466, 2-171325, 2-415581 and 4-141485.
The aforementioned conventional phase-change optical recording media are subjected to recording operation in a heat mode, so that there is observed a deteriorating tendency in the repetition of recording and erasing operations. The improvement of such repetition properties is therefore one of the most important research topics.
To improve the repetition properties, it is proposed to add a nitrogen atom to the recording layer as disclosed in Japanese Laid-Open Patent Applications 4-16383 and 8-287515. Further, as in Japanese Laid-Open Patent Application 8-287515, to improve the repetition properties, a recording layer is located so as to be interposed between a lower heat-resistant protective layer and an upper heat-resistant protective layer, and in addition, another heat-resistant protective layer is overlaid as the third heat-resistant protective layer on the upper heat-resistant protective layer. In this case, the coefficient of thermal expansion of the third heat-resistant protective layer is controlled to be smaller than those of the lower and upper heat-resistant protective layers. As a result, deformation of the recording layer, which is easily caused by the movement of elements constituting the recording layer when heat is applied to the recording medium, can be prevented.
However, the addition of a nitrogen atom to the recording material increases the crystallization temperature of the recording material. As a result, initial crystallization of the phase-change optical recording medium cannot be readily carried out, and the time required to terminate the initial crystallization is increased. Further, the provision of the third heat-resistant protective layer is disadvantageous from the manufacturing cost.